Analog dereverberation system

Abstract

Room reverberation characteristic of monaural systems is removed, in accordance with the principles of this invention, by employing two microphones at the sound source and by manipulating the signals of the two microphones to develop a single nonreverberant signal. Both early echoes and late echoes in the signal received by each microphone are removed by manipulating the signals of the two microphones in separate frequency bands of the signal. Corresponding frequency bands of the two signals are co-phased and added and the magnitude of each resulting frequency band is modified in accordance with a computed phase difference average between the corresponding frequency bands. The modified frequency bands are combined, thereby forming the nonreverberant signal.

What is claimed is: 
     
       1. An analog dereverberation system comprising: first filter/delay means for developing a first plurality of variably delayed analog bandpass signals in response to a first input signal and a control signal;   second filter/delay means for developing a second plurality of delayed analog bandpass signals in response to a second input signal;   a plurality of correlator means responsive to corresponding signals of said first and second pluralities of bandpass signals for developing a plurality of nonreverberant bandpass signals and for developing said control signal; and   means for adding said plurality of nonreverberant bandpass signals to form a single analog nonreverberant signal.   
     
     
       2. The apparatus of claim 1 wherein said first filter/delay means comprises: a bank of bandpass filters equal in number to said first plurality of bandpass signals responsive to said first input signal; and   a bank of delay elements equal in number to said first plurality of bandpass signals, with each of said delay elements being connected to the output of a different one of said bandpass filters.   
     
     
       3. The apparatus of claim 2 wherein each of said bandpass filters covers a different frequency band, and wherein said bank of bandpass filters covers the entire frequency band of said first input signal. 
     
     
       4. The apparatus of claim 3 wherein said frequency bands of said bandpass filters are equal. 
     
     
       5. The apparatus of claim 3 wherein said frequency bands of said bandpass filters overlap. 
     
     
       6. The apparatus of claim 2 wherein said delay elements comprise: Ccd shift registers; and   voltage controlled oscillator means responsive to said control signal for effecting the frequency of said oscillator means.   
     
     
       7. The apparatus of claim 1 wherein said first and said second filter/delay means each comprise: a delay element responsive to the respective input signal; and   a bank of filters, each connected to the output terminal of said delay element.   
     
     
       8. The apparatus of claim 1 wherein each correlator means of said plurality of correlator means is responsive to one preselected bandpass signal of said first filter/delay means and to a bandpass signal of said second filter/delay means having a bandpass corresponding to the bandpass of said preselected bandpass signal of said first filter/delay means comprising: a sawtooth phase detector responsive to the input signals of said correlator means for developing a signal whose average amplitude is related to the phase difference between said input signals of said correlator means;   a first low-pass filter responsive to said sawtooth phase detector for developing a signal forming a part of said control signal;   a bandpass filter responsive to said sawtooth phase detector;   a rectifier responsive to said bandpass filter;   a second low-pass filter responsive to said rectifier;   combiner means for developing a sum of said input signals of said correlator means; and   amplitude control means responsive to said second low-pass filter, for effecting the amplitude of the signal developed by said combiner means and for developing thereby a nonreverberant bandpass signal belonging to said plurality of nonreverberant bandpass signals.   
     
     
       9. An analog dereverberation system responsive to signals from two microphones comprising: filter/delay means responsive to the signals of said two microphones for time shifting the signal of one of said two microphones with respect to the signal of the other of said two microphones under control of a shift control signal and for developing a plurality of bandpass signals from the time shifted signal of said one of said two microphones and from the signal of the other of said two microphones;   means for adding corresponding bandpass signals of said one microphone and said other microphone to develop added bandpass signals;   means responsive to said plurality of bandpass signals for developing said time shift control signal and a gain control signal;   means for effecting the amplitudes of said added bandpass signals in accordance with said gain control signal to develop gain modified bandpass signals; and   means for combining said gain modified bandpass signals to form a single nonreverberant signal.